German patent specification No. 215,616 discloses an irrigator comprising a water distributing pipe situated between two driving wheels. Along the periphery these driving wheels comprise tiltable cups, which are supplied with water when they are in their uppermost position and emptied when they hit a stop in their lowest position and thereby tilt. These known cups are therefore only filled with water when they are in a particular uppermost position, which, especially when the apparatus is being started, involves an insufficient or no propellant moment. Furthermore, the mechanical tilting of the cups in their lowest position involves a sudden emptying which does not provide a necessary braking effect when the apparatus moves downhill. Consequently, the apparatus moves forwards at an uncontrollable speed, to which must be added that these tiltable cups are expensive to manufacture and to keep in repair.